The present invention pertains to the field of computer systems. More specifically, this invention relates to an input/output circuit.
With the size of many electronic products becoming increasingly smaller, many electronic product designers are currently seeking to minimize power consumption. Generally, reducing the overall magnitude of voltage swings of CMOS circuits allows a reduction in power consumption. Thus, an electronic architecture that would allow and work with lower input voltage swings without drawing leakage current is desirable. However, many applications of CMOS circuits still use high voltages, which need to be accommodated. Consequently, it is necessary to support multiple power supplies, such as Vcc and VCCQ.
A problem with supporting multiple power supplies is an unknown state during the power-up state. For example, in a high voltage level shifter, if the high voltage VCCQ powers up before the supply voltage Vcc, the output of the level shifter is undetermined. An unknown state may cause bus contentions and, accordingly, may cause the system to fail.